The simple answer to that szopen is that many introduced species such as carp and tilapia compete with native fish species for food and space, can prey upon the young of native fish, and can disturb the spawning of native species. Further, the introduction of non-native species is sometimes associated with the introduction of new fish diseases (the introduction of the fish parasite Lernea with the introduction of carp is probably a good example in Australia).Originally Posted by szopen
All that tends to amount to less native fish, and since the ecology of our waterways is 'designed' around our native fish the introduction of non-native species is not considered a good thing. Why disrupt the balance of nature if you don't have to? And who needs carp and tilapia here anyway? Yellowbelly, cod, bass etc are generally considered to be better recreational (and table) species anyway.
Mind you, some of these comments also apply to introducing Australian native species to areas of Australia where they did not occur naturally, such as the introduction of yellowbelly to southeast Qld. Some of the comments also apply to the stocking of huge numbers of predatory (native) species such as bass to natural waterways, the fish fauna of which is comprised predominantly of small native species gobbled up by all those hungry predators. I won't even go near the potential effect stocking thousands of (potentially) genetically narrow fish might have on natural populations in this conversation.
Its all about how far you are willing to push nature I guess.
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